The invention relates to a method for magnetic resonance imaging of a body placed in a stationary and substantially homogeneous main magnetic field, the method comprising repeated execution of a sequence including the following steps:
application of a first RF-pulse for excitation of at least a portion of the body, PA1 application of at least a first gradient magnetic field for phase encoding of the excited portion, PA1 application of a second RF-pulse at a time .tau. after the first RF-pulse, PA1 detection of a magnetic resonance spin-echo signal around a time 2.tau. after the first RF-pulse, PA1 whereby a repetition time between first RF-pulses in subsequent sequences is shorter than a transversal relaxation time T.sub.2 of a relevant substance of the body. With such a method, images of a slice of a human or animal body or of an object can be obtained with a spin-echo technique. By relevant substance of the body is meant a substance relevant for imaging. For diagnostic purposes, this is a substance such as a body fluid, tissue or fat. In accordance with the convention in the art, T.sub.2 is used to designate the transverse spin-spin relaxation time constant. PA1 the first gradient magnetic field is applied after the application of the second RF-pulse, and in that PA1 a second gradient magnetic field for compensation of the phase encoding caused by the first gradient magnetic field is applied in the interval after occurrence of the magnetic resonance spin-echo signal and before application of the first RF-pulse in a next sequence. Phase encoding of the excited portion of the body is introduced after the application of the 180.degree. re-phasing RF-pulse. The position dependent effects thereof are removed by the second gradient magnetic field after detection of the spin-echo signal and before the next sequence starts. Consequently, there is no phase difference left between spins in the direction of the phase encoding gradient magnetic field. Position dependent effects due to phase encoding of magnetisation in the excited portion of the body are therefore not present at the start of the next sequence, and a next excitation RF-pulse can be applied without having to realign the magnetisation along the direction of the main magnetic field. The repetition of excitation pulses prior to the vanishing of the transversal excitation allows a fast data acquisition. In medical diagnostic imaging this is most useful in breathhold abdominal imaging, where the total scan time is limited to an interval measured in seconds. PA1 a selection gradient magnetic field is applied during application of the first RF-pulse for selection of a slice of the body, and in that PA1 a third gradient magnetic field is applied during detection of the magnetic resonance spin-echo signal for frequency encoding. As known in the art, the excitation pulse in this procedure excites a slice in the body in which the Larmor frequency of the spins corresponds to the frequencies in the RF-pulse, in general the selected slice is perpendicular to the main magnetic field. Within this slice, position information in one direction is encoded by the first gradient magnetic field providing phase encoding, and in the perpendicular direction by a position dependent precession frequency caused by the third gradient magnetic field. PA1 applying a first radio-frequency pulse (RF-pulse) for excitation of at least a portion of the body, PA1 applying a at least a first gradient magnetic field for phase encoding of the excited portion, PA1 applying a second RF-pulse at a time .tau. after the first RF-pulse, PA1 detecting a magnetic resonance spin-echo signal around a time 2.tau. after the first RF-pulse, PA1 within a period that allows the repetition time TR between first RF-pulses in subsequent sequences to be shorter than a transversal relaxation time T.sub.2 of a relevant substance of the body. The apparatus according to the invention is characterised in that the control means are further arranged for
Such a method is known from EP-B 0 128 622. In that document a method for magnetic resonance imaging is disclosed in which an excitation radio-frequency pulse (90.degree. RF-pulse) is followed by a 180.degree. rephasing RF-pulse after a time interval .tau.. The 180.degree. RF-pulse rephases the transversal component of the magnetization and generates thereby a spin echo signal that is detected, providing information about the object or body. In order to encode position information in the detected signal, at least a phase encoding gradient is applied before the 180.degree. rephasing RF-pulse providing the signal with position dependent phases. These position dependent phases have to vanish before a next sequence can be started. In order to avoid waiting until the magnetisation of the excited portion has substantially returned to its equilibrium state, a second 180.degree. RF-pulse is applied after detection of the spin-echo signal causing a second spin-echo signal. A second 90.degree. RF-pulse is generated simultaneously with the occurrence of this second spin-echo signal. The second 90.degree. RF-pulse re-orientates the transverse component of the magnetisation into the direction of the main magnetic field. Consequently, it is not necessary to wait until this situation has resulted from natural relaxation of the magnetisation.
Although providing a considerable increase in sampling speed over earlier spin-echo sequences, the repetition time of this known method is still at least about 200 ms. About 100 ms of this time is needed for residual relaxation after the second 90.degree. RF-pulse is applied, and another 50 ms is needed between the detected spin-echo signal and the second 90.degree. RF-pulse. The 100 ms period in the known sequence is needed for the vanishing of position-dependent effects in the excited portion of the body, such residual position dependent effects otherwise would give rise to ghosts and artefacts in a reconstructed image. The 50 ms from the spin-echo signal to the 90.degree. RF-pulse cannot be reduced significantly as the second 180.degree. RF-pulse cannot be applied before the end of detection sequence of the spin echo signal. Therefore, it does not seem feasible to achieve a significant reduction of the repetition time in the known method by reduction of the intervals in between RF-pulses.